Sacrificial layer for channel surface retention and inner spacer formation in stacked-channel FETs

ABSTRACT

Field effect transistors include a stack of nanosheets of vertically arranged channel layers. A gate stack is formed over, around, and between the vertically arranged channel layers. Spacers are formed, with at least one top pair of spacers being positioned above an uppermost channel layer. The top pair of spacers each has a curved lower portion with a curved surface in contact with the gate stack and a straight upper portion that extends vertically from the curved portion along a straight sidewall of the gate stack.

BACKGROUND Technical Field

The present invention relates to stacked-channel field effecttransistors and, more particularly, to the use of a sacrificial layer toimprove retention of top channel layers.

Description of the Related Art

Stacked nanowire and nanosheet devices are proposed for modern andnext-generation semiconductor devices. The devices are formed aroundthin channel materials, configured as either wires or sheets ofsemiconductor material. Parasitic capacitance in nanosheet devices is asignificant performance detractor, which can be mitigated through theformation of an inner spacer between a gate stack and the source anddrain regions.

However, the dummy gate processes used to form stacked nanowire andnanosheet devices can cause significant damage to the small-scalechannel structures. In particular, etching processes used to removedummy gate structures and form inner spacers are not perfectly selectiveand damages the top layer of a nanowire or nanosheet channel. In manycases, the top layer of a channel is destroyed entirely by the process.

SUMMARY

A field effect transistor includes a stack of nanosheets of verticallyarranged channel layers. A gate stack is formed over, around, andbetween the vertically arranged channel layers. Spacers are formed, withat least one top pair of spacers being positioned above an uppermostchannel layer. The top pair of spacers each has a curved lower portionwith a curved surface in contact with the gate stack and a straightupper portion that extends vertically from the curved portion along astraight sidewall of the gate stack.

A field effect transistor includes a stack of nanosheets of verticallyarranged channel layers. Merged source and drain regions are positionedat respective ends of the vertically arranged channel layers. Eachmerged source and drain region contacts multiple channel layers. A gatestack is formed over, around, and between the vertically arrangedchannel layers. A pair of top spacers is positioned above an uppermostchannel layer. Each top spacer has a crescent-shaped lower portion witha curved surface in contact with the gate stack and a straight upperportion that extends vertically from the curved portion along a straightsidewall of the gate stack.

A field effect transistor includes a stack of nanosheets of verticallyarranged channel layers. A top channel layer of the stack of nanosheetsis not damaged from an anisotropic etch. A gate stack is formed over,around, and between the vertically arranged channel layers. Spacers areformed, with at least one top pair of spacers being positioned above anuppermost channel layer. The top pair of spacers each have a curvedlower portion with a curved surface in contact with the gate stack and astraight upper portion that extends vertically from the curved portionalong a straight sidewall of the gate stack.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 2 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 3 is a top-down diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 4 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 5 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 6 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 7 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 8 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 9 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 10 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 11 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 12 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 13 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 14 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 15 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 16 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles;

FIG. 17 is a cross-sectional diagram of a step in forming a field effecttransistor in accordance with the present principles; and

FIG. 18 is a block/flow diagram of a method of forming a field effecttransistor in accordance with the present principles.

DETAILED DESCRIPTION

Embodiments of the present invention employ a layer of epitaxially grownsacrificial material on a top layer of a nanosheet channel. Thissacrificial layer protects the top layer from subsequent etches andprevents damage to the channel material. In addition, the presence ofthe sacrificial layer leads to the formation of an inner spacer abovethe uppermost channel layer.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a step in the formation ofa field effect transistor (FET) is shown. In the present embodiments, asemiconductor-on-insulator (SOI) embodiment is shown. Specifically, theSOI structure includes a bottom semiconductor layer in the form of,e.g., a silicon substrate 100. An insulating layer 102 lies over thesubstrate 100 and may be formed from, e.g., silicon dioxide. Thesemiconductor layer 100 may include any appropriate semiconductormaterial including, e.g., silicon, silicon germanium, silicon germaniumcarbide, silicon carbide, polysilicon, epitaxial silicon, amorphoussilicon, and multilayers thereof. Although silicon is the predominantlyused semiconductor material in wafer fabrication, alternativesemiconductor materials can be employed, such as, but not limited to,germanium, gallium arsenide, gallium nitride, silicon germanium, cadmiumtelluride and zinc sellenide. The insulating layer may be a buried oxideregion formed from, e.g., silicon dioxide, but it should be understoodthat any suitable dielectric material may be used. Exemplary alternativedielectric materials which may be used include, e.g., hafnium oxide,zirconium oxide, lanthanum oxide, aluminum oxide, titanium oxide,strontium titanium oxide, lanthanum aluminum oxide, yttrium oxide,hafnium oxynitride, zirconium oxynitride, lanthanum oxynitride, aluminumoxynitride, strontium titanium oxynitride, lanthanum aluminumoxynitride, yttrium oxynitride, silicates of the above, and alloys ofthe above.

On top of the insulating layer 102 is a stack 108 of semiconductormaterials. The stack includes alternating layers of sacrificial material104 and channel material 106. In one particular embodiment, it isspecifically contemplated that the sacrificial material 104 may beformed from, e.g., silicon germanium, while the channel material 106 maybe formed from, e.g., silicon. While it is specifically contemplatedthat the sacrificial material 104 may be silicon germanium, it should beunderstood that any material may be used that has etch selectivity withthe channel material 106, such that the sacrificial material 104 may beremoved without harming structures made from the channel material 106.

Suitable materials for the channel material 106 include, for example,silicon, silicon germanium, silicon germanium carbide, silicon carbide,polysilicon, epitaxial silicon, amorphous silicon, multi-layers thereof,germanium, gallium arsenide, gallium nitride, silicon germanium, cadmiumtelluride, and zinc sellenide. III-V semiconductors may alternatively beused for the channel material 106. The term “III-V compoundsemiconductor” denotes a semiconductor material that includes at leastone element from Group III of the Periodic Table of Elements and atleast one element from Group V of the Periodic Table of Elements.Typically, the III-V compound semiconductors are binary, ternary orquaternary alloys including III/V elements. Examples of III-V compoundsemiconductors that can be used in the present embodiments include, butare not limited to alloys of gallium arsenic, aluminum arsenic, indiumgallium arsenic, indium aluminum arsenic, indium aluminum arsenicantimony, indium aluminum arsenic phosphorus, indium gallium arsenicphosphorus and combinations thereof.

It is specifically contemplated that the stack 108 may be epitaxiallygrown. This refers to the growth of a semiconductor material on adeposition surface of a semiconductor material, in which thesemiconductor material being grown has substantially the samecrystalline characteristics as the semiconductor material of thedeposition surface. The term “epitaxial material” denotes a materialthat is formed using epitaxial growth. In some embodiments, when thechemical reactants are controlled and the system parameters setcorrectly, the depositing atoms arrive at the deposition surface withsufficient energy to move on the surface and orient themselves to thecrystal arrangement of the atoms of the deposition surface. Thus, insome examples, an epitaxial film deposited on a {100} crystal surfacewill take on a {100} orientation.

In the case of a silicon channel material 106, the silicon gas sourcefor epitaxial deposition may be selected from the group consisting of,e.g., hexachlorodisilane, tetrachlorosilane, dichlorosilane,trichlorosilane, methylsilane, dimethylsilane, ethylsilane,methyldisilane, dimethyldisilane, hexamethyldisilane and combinationsthereof. In the case of a silicon germanium sacrificial material, thesilicon sources for epitaxial deposition may be selected from the groupconsisting of silane, disilane, trisilane, tetrasilane,hexachlorodisilane, tetrachlorosilane, dichlorosilane, trichlorosilane,methylsilane, dimethylsilane, ethylsilane, methyldisilane,dimethyldisilane, hexamethyldisilane and combinations thereof, and thegermanium gas sources may be selected from the group consisting ofgermane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof.

Referring now to FIG. 2, a step in the formation of a FET is shown. Inthis step, the stack 108 is etched to form fins 202 (having a lengthperpendicular to the cross-section shown. Any appropriate anisotropicetch may be used to form the fins 202, with material being removed inthe direction normal to the surface of the stack 108 at a rate that isgreater than in the direction parallel to the surface. Although narrowfins 202 are shown, any width of fin may be used to create structureshaving any width from nanowires to nanosheets. It should be understoodthat a nanowire has a cross-sectional height that is on the same orderof magnitude as its cross-sectional width (in one specific embodiment, ananowire has a ratio of cross-sectional width to cross-sectional heightof less than 2:1), whereas a nanosheet has a cross-sectional height thatis significantly smaller than its cross-sectional width (in one specificembodiment, having a ratio of cross-sectional width to cross-sectionalheight of greater than 2:1).

In one exemplary embodiment, a reactive ion etch (RIE) may be used toform the fins 202. RIE is a form of plasma etching in which, duringetching, the surface to be etched is placed on an RF powered electrode.During RIE, the surface to be etched takes on a potential thataccelerates the etching species' extracted from plasma toward thesurface, with the chemical etching reaction taking place in thedirection normal to the surface. Other examples of anisotropic etchingthat can be used include ion beam etching, plasma etching or laserablation. Alternatively, the fin structure 202 can be formed by spacerimaging transfer.

The etch is selective to the material of the insulator layer 102. Asused herein, the term “selective” in reference to a material removalprocess denotes that the rate of material removal for a first materialis greater than the rate of removal for at least another material of thestructure to which the material removal process is being applied. Inthis case, the etch is performed in a manner that is selective betweenthe stack 108 of sacrificial material 104 and channel material 106.

Referring now to FIG. 3, a top-down view of the step of FIG. 2 is shown.The fins 202 shown as being disposed on the insulator layer 102 atregular intervals, but it should be understood that any number of fins202 may be used at any fixed or varying spacing. Also shown with dashedline A is a cross-section to be used for the following figures anddiscussion.

Referring now to FIG. 4, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. A set of dummygates is deposited, with two outside dummy gates 402 and a central dummygate 404. The dummy gates 402 and 404 may be formed from a semiconductormaterial including, e.g., polysilicon or amorphous silicon, that hasetch selectivity with the sacrificial material 104. The dummy gates 402and 404 cover the ends of the fins 202 and the center of the fins 202,while leaving the areas between exposed. The dummy gates 402/404 may beformed by depositing material over the fins 202 and subsequentlypatterning the into the dummy gate material using any appropriatelithography or etch process.

Referring now to FIG. 5, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. Spacers 502 aredeposited conformally along the sidewalls of the gates 402 and 404.Spacer material 502 may be removed from the sidewalls of the fins 202using an anisotropic etch such as RIE. Note that the present figures arenot drawn exactly to scale—the removal of the spacer material 502 fromthe sidewalls of the fins 202 will result in an etch back along thesidewalls of the dummy gates 404 at least as deep as the height of thefins 202. Deposition of the spacers 502 may include any appropriatedeposition process including, e.g., chemical vapor deposition (CVD),physical vapor deposition, or atomic layer deposition. In one particularembodiment, the spacers 502 may be formed from a hardmask material suchas, e.g., silicon nitride. Other hardmask compositions for the spacers502 include silicon oxides, silicon oxynitrides, silicon carbides,silicon carbonitrides, etc. Spin-on dielectrics may also be utilized asa hardmask material including, but not limited to: silsequioxanes,siloxanes, and boron phosphate silicate glass.

CVD is a deposition process in which a deposited species is formed as aresult of chemical reaction between gaseous reactants at greater thanroom temperature (e.g., from about 25° C. to about 900° C.). The solidproduct of the reaction is deposited on the surface on which a film,coating, or layer of the solid product is to be formed. Variations ofCVD processes include, but not limited to, Atmospheric Pressure CVD, LowPressure CVD and Plasma Enhanced CVD, Metal-Organic CVD, andcombinations thereof. PVD may include processes such as sputteringwhere, for example, DC diode systems, radio frequency sputtering,magnetron sputtering, and ionized metal plasma sputtering.

Referring now to FIG. 6, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. Portions of thefins 202 are etched away, leaving a channel region 602 under the centraldummy gate 404, with remnants 604 embedded in the lateral dummy gates402. An anisotropic etch, such as RIE, is used to remove the material inthe regions that will eventually become the source and drain of thedevice.

Referring now to FIG. 7, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. A selective,timed, isotropic etch is used to remove some of the sacrificial material104, recessing the sacrificial material 104 with respect to the channelmaterial 106. This results in recessed sacrificial material 704 andprotruding channel material 702. The term “isotropic etch” denotes anetch process that is non-directional. By “non-directional” it is meantthat the etch rate is not substantially greater in any one direction incomparison to all of the etch directions. The etch is timed, with aknown etching rate being used to determine how long to allow the etch tocontinue to achieve a predetermined degree of recess in the recessedsacrificial material 704. One notable feature of the recessedsacrificial material 704 is emphasized in the dashed box 706, shown ingreater detail in FIG. 8 below.

The isotropic etch may be a wet chemical etch or a dry etch. Forexample, the etchant may be a corrosive liquid or a chemically activeionized gas, such as a plasma. The precise composition of the etch willdepend on the character of the sacrificial material 104 and the channelmaterial 106, with the etch selectively removing only the sacrificialmaterial. For example, wet etches may include inorganic acids andoxidizing agents that do not attack the channel material 106 may beused. Examples of oxidizing agents may include peroxides, nitrates,nitrites, perchlorates, chlorates, chlorites, hypochlorites,dichromates, permanganates, persulfates or combinations thereof. Theinorganic acids can include hydrochloric acid, hydrofluoric acid,sulfuric acid, phosphoric acid, or combinations thereof.

Referring now to FIG. 8, an expanded view of box 706 is shown. Inparticular, detail on the recessed layers of sacrificial material 704 isshown. The etch that recessed the sacrificial material 704 is notperfectly isotropic, leaving a curved shape 802 to the outward facingsurface of the recessed sacrificial material 704.

Referring now to FIG. 9, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. In this step, thespacers 502 are removed using an isotropic etch. As above, the isotropicetch may be a wet chemical etch or a dry etch. The removal of thespacers 502 exposes the sidewalls of the dummy gates 402 and 404.

Referring now to FIG. 10, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. A layer of spacermaterial is conformally deposited over the surfaces of the gates402/404, the recessed sacrificial material 704, and the protrudingchannel material 702. As above, deposition of the spacers 1004 mayinclude any appropriate deposition process including, e.g., chemicalvapor deposition (CVD), physical vapor deposition, or atomic layerdeposition. In one particular embodiment, the spacers 704 may be formedfrom a hardmask material such as, e.g., silicon nitride. Other hardmaskcompositions for the spacers 704 include silicon oxides, siliconoxynitrides, silicon carbides, silicon carbonitrides, etc. Spin-ondielectrics may also be utilized as a hardmask material including, butnot limited to: silsequioxanes, siloxanes, and boron phosphate silicateglass.

An anisotropic etch, such as RIE, is used to remove spacer material fromthe facing surfaces of the protruding channel material 702, exposing aface of the channel material. The resulting surface is emphasized in thedashed box 1002 and shown in greater detail below.

Referring now to FIG. 11, an expanded view of box 1002 is shown. Inparticular, detail on the spacer 1004 is shown. In particular, afterremoval of the material on the facing surface of the protruding channelmaterial 702, spacer material 1102 remains in the recessed portions ofthe recessed sacrificial material 704. In practice, these spacers 1102form “crescent moon” shapes that remain as a structural component of thefinished device. In contrast to conventional devices, the presence of alayer of sacrificial material 704 on top of the uppermost layer ofprotruding channel material 702 causes one such spacer 7702 to formabove the uppermost layer of protruding channel material 702.

Referring now to FIG. 12, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. Source and drainregions are epitaxially grown from the exposed faces of the channelmaterial 702, with vertically adjacent channel layers merging to produceactive source/drain regions 1202 and vestigial source/drain regions1204. In the case of a silicon channel material 702, the silicon gassource for epitaxial deposition may be selected from the groupconsisting of, e.g., hexachlorodisilane, tetrachlorosilane,dichlorosilane, trichlorosilane, methylsilane, dimethylsilane,ethylsilane, methyldisilane, dimethyldisilane, hexamethyldisilane andcombinations thereof. As shown, growth follows a crystalline growthpattern that follows the crystalline structure of the facing surface ofthe protruding channel material 702. The vestigial merged source/drainregions 1204 are emphasized in dashed box 1206 and shown in FIG. 13below.

The source/drain regions 1202/1204 may be doped with dopant atoms. Thedopant atoms may be an n-type dopant (i.e., an element from Group IV orVI of the Periodic Table of Elements) or a p-type dopant (i.e., anelement from Group II or VI of the Periodic Table of Elements).Exemplary n-type dopants for a group IV semiconductor includephosphorus, arsenic and antimony. Exemplary p-type dopants for a groupIV semiconductor include boron, aluminum, and gallium. Exemplary n-typedopants for a III-V semiconductor include selenium, tellurium, silicon,and germanium. Exemplary p-type dopants for a III-V semiconductorinclude beryllium, zinc, cadmium, silicon, and germanium. Theconcentration of dopant within the doped region is typically from about1011 to about 1015 atoms/cm2, with a concentration of dopant within thedoped region from about 1011 to about 1013 atoms/cm2 being more typical.The source/drain regions 1202/1204 may be in situ doped as they aregrown on the channel material 702 or, alternatively, may be dopedthrough an implantation process. Notably, the channel material 702 thatremains underneath the dummy gates 402/404 is not doped.

Referring now to FIG. 13, an expanded view of box 1206 is shown. Inparticular, detail on the merged source/drain region 1204 is shown. Itshould be understood that the active source/drain regions 1202 and thevestigial source/drain regions 1204 are essentially similar instructure. The vertically adjacent channel layers merge and become asingle crystalline structure, with the crescent moon spacers 1102trapped within.

Referring now to FIG. 14, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. Dielectricmaterial 1402 is filled to or above the level of the top surface of thedummy gates 402/404. It is specifically contemplated that the dielectricmaterial 1402 may have the same composition as the insulator layer 102or may, alternatively, be formed from any appropriate insulatingmaterial. If the dielectric material 1402 is filled to a level above thetop surface of the dummy gates 402/404, the material may then bepolished down to the level of the dummy gates 402/404 using a chemicalmechanical planarization that stops on the material of the dummy gates402/404.

Referring now to FIG. 15, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. The dummy gates402/404 have been removed, exposing the remaining sacrificial material704. An anisotropic etch is used to remove the dummy gate material, butit should be recognized that no anisotropic etch is perfectly selective.This etch will cause some damage to the top layer of the sacrificialmaterial 704—the presence of the sacrificial material 704 protects theunderlying channel layers from damage.

Referring now to FIG. 16, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. An isotropic etchis used to remove the remaining sacrificial material 704, leavingchannels 1602 exposed. The etch may be a wet etch or a dry etch. Forexample, wet etches may include inorganic acids and oxidizing agentsthat do not attack the channel material 106 may be used. Examples ofoxidizing agents may include peroxides, nitrates, nitrites,perchlorates, chlorates, chlorites, hypochlorites, dichromates,permanganates, persulfates or combinations thereof. The inorganic acidscan include hydrochloric acid, hydrofluoric acid, sulfuric acid,phosphoric acid, or combinations thereof.

Referring now to FIG. 16, a cross-sectional view of a step in theformation of a FET is shown along the cross-section A. A gate stack 1702is formed, including for example a gate dielectric layer, a workfunction metal, and a gate material that may be deposited and planarizedaccording to any appropriate process. The device may be finished byforming electrical contacts to the active source/drain regions 1202 andthe gate stack 1702.

It is to be understood that the present invention will be described interms of a given illustrative architecture having a wafer; however,other architectures, structures, substrate materials and processfeatures and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to FIG. 18, a block/flow diagram of a method of forming aFET is shown. Block 1802 forms the stack 108 of layers of sacrificialmaterial 104 and layers of channel material 106, with a layer ofsacrificial material at the top of the stack 108. Block 1802 etches fins202 into the stack 108, with each fin 202 providing the basis for a FETand being spaced apart in accordance with design needs and technologylimits.

Block 1806 forms dummy gates 402 and 404 over the spacers, depositing alayer of material and etching away any material that does not belong tothe dummy gates 402/404 using, e.g., photolithography or an anisotropicetch. Block 1807 forms spacers 502 on the sidewalls of the dummy gates402/404 by, e.g., conformally depositing a layer of spacer material andanisotropically etching them material from horizontal surfaces. Block1808 then anisotropically etches the fins 202 from areas around thedummy gates 402/404, leaving three fin regions remaining underneath thedummy gates 402/404.

Block 1810 uses an isotropic etch, for example a wet chemical etch, toselectively recess the sacrificial material and create recessedsacrificial layers 704 with recesses 802. Block 1812 removes theremaining spacer material from the first spacers 502 and block 1814forms a second layer of spacers material conformally over the recessedsacrificial layers 704. Block 1816 anisotropically etches the secondspacers 1004, removing any spacer material on horizontal surfaces orwhich extends past the protruding channel layers 702 to form the innerspacers 1102.

Block 1818 epixtaxially grows source/drain regions 1202/1204 from theexposed faces of the protruding channel layers 704. Block 1820 formsdielectric material 1402 around the dummy gates 402/404 to or above thelevel of the top surface of the dummy gates 402/404. Block 1822 etchesaway the dummy gates with an anisotropic etch to expose the remainingsacrificial material 704. The remaining sacrificial material 704protects the underlying channel layers 1602 from the etch that removesthe dummy gates 402/404. Block 1824 removes the remaining sacrificialmaterial 704 to expose the channel layers 1602. Block 1826 then formsthe gate stack 1702 over and around the channel layers 1602.

Having described preferred embodiments of a sacrificial layer forchannel surface retention and inner spacer formation in stacked-channelFETs (which are intended to be illustrative and not limiting), it isnoted that modifications and variations can be made by persons skilledin the art in light of the above teachings. It is therefore to beunderstood that changes may be made in the particular embodimentsdisclosed which are within the scope of the invention as outlined by theappended claims. Having thus described aspects of the invention, withthe details and particularity required by the patent laws, what isclaimed and desired protected by Letters Patent is set forth in theappended claims.

The invention claimed is:
 1. A field effect transistor, comprising: astack of nanosheets of vertically arranged channel layers; a gate stackformed over, around, and between the vertically arranged channel layers;and a plurality of spacers, with at least one top pair of spacers beingpositioned above an uppermost channel layer, the at least one top pairof spacers each comprising a curved lower portion with a curved surfacein contact with the gate stack and a straight upper portion that extendsvertically from the curved lower portion along a straight sidewall ofthe gate stack.
 2. The field effect transistor of claim 1, wherein theplurality of spacers each have a crescent shape.
 3. The field effecttransistor of claim 2, wherein the source and drain regions have bulgescomplementary to recesses in the crescent-shaped spacers.
 4. The fieldeffect transistor of claim 2, wherein the gate stack is recessed toaccommodate a crescent shape of the topmost pair of spacers.
 5. Thefield effect transistor of claim 1, wherein each of the stack ofnanosheets has a cross-sectional width that is significantly greaterthan a cross-sectional height.
 6. The field effect transistor of claim1, wherein a top channel layer of the stack of nanosheets is not damagedfrom an anisotropic etch.
 7. The field effect transistor of claim 6,wherein the top channel layer has a uniform thickness between portionsthat are between the top pair of spacers and a lower pair of spacers andportions that are in the gate stack.
 8. A field effect transistor,comprising: a stack of nanosheets of vertically arranged channel layers;merged source and drain regions at respective ends of the verticallyarranged channel layers, wherein each merged source and drain regioncontacts multiple channel layers; a gate stack formed over, around, andbetween the vertically arranged channel layers; and a pair of topspacers positioned above an uppermost channel layer, each comprising acrescent-shaped lower portion with a curved surface in contact with thegate stack and a straight upper portion that extends vertically from thecurved portion along a straight sidewall of the gate stack.
 9. The fieldeffect transistor of claim 8, wherein the merged source and drainregions share a crystalline structure with the channel layers.
 10. Thefield effect transistor of claim 9, wherein the each merged source anddrain region is epitaxially grown from a respective set of ends of thechannel layers.
 11. The field effect transistor of claim 10, wherein thesource and drain regions have bulges complementary to recesses in thecrescent-shaped internal spacers.
 12. The field effect transistor ofclaim 10, wherein the gate stack is recessed to accommodate a crescentshape of the topmost pair of spacers.
 13. The field effect transistor ofclaim 8, further comprising vestigial source and drain regions formed inlateral gate stacks on each side of the gate stack.
 14. The field effecttransistor of claim 8, wherein each of the stack of nanosheets has across-sectional width that is significantly greater than across-sectional height.
 15. The field effect transistor of claim 8,wherein a top channel layer of the stack of nanosheets is not damagedfrom an anisotropic etch.
 16. The field effect transistor of claim 15,wherein the top channel layer has a uniform thickness between portionsthat are between the top pair of spacers and a lower pair of spacers andportions that are in the gate stack.
 17. A field effect transistor,comprising: a stack of nanosheets of vertically arranged channel layers,wherein a top channel layer of the stack of nanosheets is not damagedfrom an anisotropic etch; a gate stack formed over, around, and betweenthe vertically arranged channel layers; and a plurality of spacers, withat least one top pair of spacers being positioned above an uppermostchannel layer, the top pair of spacers each comprising a curved lowerportion with a curved surface in contact with the gate stack and astraight upper portion that extends vertically from the curved portionalong a straight sidewall of the gate stack.
 18. The field effecttransistor of claim 17, wherein the spacers each have a crescent shape.19. The field effect transistor of claim 18, wherein the source anddrain regions have bulges complementary to recesses in thecrescent-shaped spacers.
 20. The field effect transistor of claim 17,wherein the top channel layer has a uniform thickness between portionsthat are between the top pair of spacers and a lower pair of spacers andportions that are in the gate stack.